The detection of kestoses provides a biochemical differentiation between race 3 and races 0 and 1 of P. parasitica var. nicotianae, the causal agent of tobacco black shank. Races 0 and 1 produce kestoses (fructosylsucrose) when grown for 5 days on a medium containing 3% sucrose. Under these conditions race 3 of this pathogen does not produce kestoses. P. cactorum, P. cambivora, P. cinnamomi (from avocado), P. citricola, P. citrophthora and P. parasitica produce kestoses but P. capsici and P. cinnamomi (from pineapple) do not.